1. Field of the Invention
The present invention relates generally to a frequency modulator (hereinafter referred to as an FM modulator), and more particularly, to an FM modulator for generating a high-frequency and wide band frequency modulation signal by an optical signal processing technique.
2. Description of the Background Art
FIG. 4 is a block diagram showing the general construction of a conventional FM modulator. In FIG. 4, the conventional FM modulator comprises a signal source 101, a frequency modulation laser (hereinafter referred to as an FM laser) 102, a first optical waveguide portion 103, a local light source 104, a second optical waveguide portion 105, and an optical detecting portion 106. The construction and the operations of the conventional FM modulator will be described in detail.
The signal source 101 outputs an electrical signal which is the original signal to be frequency-modulated. The FM laser 102, which is composed of a laser diode, for example, oscillates light having a wavelength .lambda.1 in a steady state condition so that an injection current to the FM laser 102 is constant. When the injection current to the FM laser 102 is amplitude-modulated by the electrical signal outputted from the signal source 101, its wavelength (or its optical frequency) is modulated. Thereby, FM laser 102 outputs an optical frequency modulation signal having the wavelength .lambda.1 at the center. And then, an optical intensity of the outputted signal from the FM laser 102 is simultaneously modulated as well as its wavelength. The first optical waveguide portion 103 guides the optical frequency modulation signal outputted from the FM laser 102. The local light source 104 outputs light having a wavelength .lambda.0 which differs by a predetermined amount .DELTA..lambda. A from the wavelength .lambda.1 of the FM laser 102. The second optical waveguide portion 105 guides the unmodulated light from the local light source 104. Each light guided by the first and second optical waveguide portions 103 and 105 is inputted to the optical detecting portion 106.
The optical detecting portion 106, which is composed of a photodiode operating the square-law detection, for example, has the properties of converting an optical intensity modulation component of the inputted light into a current amplitude modulation component (hereinafter referred to as an IM-DD component) and, generating, when two lights having different optical wavelengths are inputted to the optical detecting portion 106, a beat component at a frequency corresponding to the difference in wavelength between the two lights (this operation is referred to as a heterodyne detection). Consequently, the optical detecting portion 106 outputs, when the outputted optical signal from the FM laser 102 and the outputted light from the local light source 104 are inputted thereto, a beat signal at a frequency corresponding to the difference in wavelength .DELTA..lambda. between the two lights. The beat signal is an FM modulation signal whose original signal is the outputted signal from the signal source 101.
As described in the foregoing, when suitable FM laser 102 and suitable local light source 104 are used in the FM modulator which converts an electrical signal into a frequency modulation signal by utilizing such properties that the wavelength of the laser diode or the like changes according to the injection current thereto(hereinafter referred to as wavelength chirping) and the heterodyne detection, an FM modulation performance having a higher frequency (not less than several GHz) and having a larger amount of frequency deviation (not less than several hundred MHz) can be realized, as compared with that in a system using a normal electric devices. Consequently, it is possible to also modulate a wide band signal such as a multi-channel frequency division multiplex signal used in CATV transmission or the like.
Although an AM (amplitude modulation) transmission method is currently used most commonly in the CATV, the method requires very good noise characteristics (for example, SNR: not less than 51 dB) on a transmission system. On the other hand, an FM (frequency modulation) transmission method has the advantage that it does not require noise characteristics as good as that of the AM transmission method, although it requires a wider transmission band. If an AM-FDM (amplitude modulation-frequency division multiplex) signal currently used in the CATV is transmitted after being simultaneously converted into an FM modulation signal using the FM modulator of the above-mentioned construction, an AM signal inherently requiring a high signal-to-noise ratio (SNR) can be transmitted using a transmission path which is not superior in noise characteristics (see a document; K. Kikushima et.al. "150-km Non-Repeated 60-Channel AM-Video Transmission Employing Optical Heterodyne AM/FM Converter", ECOC '95, Th.L. 3.1, Brussels, 1995, for example).
In the FM modulator shown in FIG. 4, the generated FM modulation signal has an amplitude variation and an average value variation due to an optical intensity modulation component produced in the FM laser 102. The amplitude variation and the average value variation cause waveform distortion at the demodulation of the FM modulation signal. The foregoing will be specifically described.
FIG. 5(a), 5(b) and 5(c) are schematic diagrams for explaining the relationship between the spectrum and the waveform of a signal in each portion of the FM modulator shown in FIG. 4. When the optical signal outputted from the FM laser 102 has only an optical frequency modulation component, an ideal FM modulation signal having no amplitude variation and no average value variation is generated, as shown in FIG. 5(a). That is to say, the spectrum of the heterodyne detection component has a shape, which is bilaterally symmetrical about a center of frequency fc corresponding to the difference in wavelength (.lambda.1-.lambda.0) between the FM laser 102 and the local light source 104, like the ideal FM modulation signal.
On the other hand, as described above, when the optical signal outputted from the FM laser 102 has both an optical intensity modulation(IM modulation) component and an optical frequency modulation component at the same time, the spectrum of a heterodyne detection component has a shape which is bilaterally asymmetrical, and the waveform thereof has an amplitude variation (an IM modulation component), as shown in FIG. 5(b) (such a signal is hereinafter referred to as an FM+IM signal).
Furthermore, in the FM modulator, an IM-DD component, that is to say, a component produced by a square-law detecting the optical intensity modulation component in the optical detecting portion 106 is superimposed on the FM+IM signal. Therefore, the FM modulator outputs an FM+IM signal whose average value (or low frequency component) varies depending on the IM-DD component as an FM modulation signal, as shown in FIG. 5(c).
Description is now made of an FM demodulation circuit for demodulating the wide band FM modulation signal generated by the FM modulator. FIG. 6 is a block diagram showing the typical construction of such an FM demodulation circuit. In FIG. 6, the FM demodulation circuit comprises a discrimination portion 601, a delay portion 602, an AND gate 603, and a low-pass filter (hereinafter referred to as LPF) 604. In FIG. 6, a signal waveform in each portion in a case where the ideal FM modulation signal (see FIG. 5(a)) is inputted to the FM demodulation circuit is together illustrated.
The discrimination portion 601, which is composed of a logic element such as an AND gate, discriminates the inputted FM modulation signal with a predetermined threshold value Vref and converts the FM modulation signal into a pulse signal (a logical signal). Further, the discrimination portion 601 has two output ports for outputting two branched pulse signals. The pulse signal outputted from one of the output ports is directly inputted to the AND gate 603. The pulse signal outputted from the other output port, after being delayed by a predetermined amount of propagation delay through the delay portion 602, is inputted to the AND gate 603. The AND gate 603 generates an AND signal of the inputted two pulse signals. The LPF 604 passes only a predetermined low frequency component corresponding to an occupied frequency band of the electrical signal outputted from the above-mentioned signal source 101, thereby to obtain an FM demodulation signal.
An amplitude variation component of the FM demodulation signal obtained as described above uniquely corresponds to a frequency variation component of the FM modulation signal. As already described, however, it is impossible to generate an ideal FM modulation signal in the FM modulator. The FM modulation signal has an amplitude variation (an IM variation component) and an average value variation (an IM-DD component) (see FIG. 5 (c)). The amplitude variation component can be removed when the FM modulation signal is converted to the pulse signal through the discrimination portion 601 of the FM demodulation circuit, while the average value variation component cannot be removed by the discrimination through the discrimination portion 601. That is to say, in such a structure that the FM modulation signal is discriminated at a fixed threshold value Vref as in the FM demodulation circuit shown in FIG. 6, accurate discrimination is difficult, so that the waveform of the FM demodulation signal is distorted, as also apparent from FIG. 7.
As described in the foregoing, the FM modulator utilizing an optical frequency modulating operation of the laser diode and heterodyne detection has a wider band frequency modulation performance, as compared with that of an FM modulator using an electrical devices, and is in prospect as a system for making it possible to perform simultaneous frequency modulation of a multi-channel signal. However, an amplitude variation and an average value variation occur in the FM modulation signal produced in that way. Therefore, it is difficult to perform an accurate FM demodulation and the quality of the demodulation signal degrades easily.